This invention relates to tire treads and more particularly to tire treads having a plurality of zig-zag ribs separated by parallel zig-zag grooves with varying depths along their lengths.
Tires, especially truck tires with circumferential zig-zag ribs, have been observed to be susceptible to a type of uneven wear generally referred to as "river wear". This type of wear which can be observed on trucks running on highways, is characterized in that the protruding portions of the ribs wear faster than the recessed portions of the ribs. It is generally believed that this type of wear is caused by excess stresses localized at the protruding portions of the ribs. It is furthermore believed that by making the rib peaks more flexible as compared to the rib corners, the uneven wear can be reduced to a great extent. This invention provides the desired localized flexibility by varying the depth of the zig-zag grooves in the circumferential direction of the tire such that they are deepest at the vertices of the projecting angles of the ribs and shallowest at the vertices of the reentrant angles of the ribs.
There is provided in accordance with the invention a tire having a treat with a pair of circumferentially extending ribs separated by a circumferentially extending zig-zag groove, said zig-zag groove having a pair of walls, respectively associated with said pair of ribs, and a base, each wall connecting the base of the groove with the ground-engaging surface of the respective rib, each rib having a series of alternately projecting and reentrant angles with the vertices of said angles located at points where the respective groove walls make changes in direction with respect to the axis of rotation of said tire, characterized in that throughout the useful life of the tire the base of said groove comprises a surface which is disposed at a radially measured distance from the ground-engaging surfaces of the ribs which varies in the circumferential direction of the tire and which distance is at a maximum at the vertex of each of said projecting angles and at a minimum at the vertex of each of said reentrant angles.